The use of quadrature amplitude modulation ("QAM") and vestigial sideband ("VSB") modulation for transmitting data, such as high definition television ("HDTV") signals, is growing in popularity.
In the United States, Advanced television ("ATV") systems, which include, e.g., HDTV systems, appear to be headed in two different directions with regard to the type of modulation used to transmit television signals.
At present, cable companies in the United States are preparing to transmit digital signals over cable using a QAM technique of data transmission.
On the other hand, the terrestrial television industry in the U.S. is planning to broadcast digital HDTV, e.g., ATV signals, using a Vestigial SideBand ("VSB") technique of data transmission proposed by Zenith corporation/Grand Alliance.
Accordingly, to be capable of receiving and displaying both cable and terrestrial HDTV television transmissions, a television receiver may have to be capable of receiving and demodulating both QAM and VSB HDTV signals. In order to reduce the cost of implementing such televisions, it is desirable that the amount of common circuitry used for demodulating VSB and QAM signals be maximized.
Nyquist filters are often used as part of a demodulator. In order to provide demodulation of multiple signal formats, e.g., VSB and QAM formats, in the most cost effective manner, a single demodulator should be able to handle either signal format with a minimum of extra hardware. Accordingly, there is a need for a Nyquist/matched filter which can be used with both VSB and QAM signals. In addition, there is a need for such a filter be capable of being implemented with little or no additional cost over the cost of a single format Nyquist filter.